1. Field of the Invention
The invention generally relates to the field of communications. More specifically the invention is related to interference suppression for use in coded signal communications, such as Code Division Multiple Access (“CDMA”) communications.
2. Discussion of the Related Art
Interference in communications obstructs the intended reception of a signal and is a persistent problem. Interference may exist in many forms. In CDMA communications, for example, interference is typically the result of receiving one or more unwanted signals simultaneously with a selected signal. These unwanted signals may disrupt the reception of the selected signal because of mutual interference. This disruption of the selected signal is typical in CDMA telephony systems and may corrupt data retrieval processes of a selected signal.
In CDMA telephony, a communications system typically includes a plurality of “base stations” providing a coverage area within a geographic region. These base stations communicate with mobile telephones and/or other CDMA devices operating within the coverage area. To illustrate, a base station provides a coverage “cell” within the overall communication coverage area maintained by the communications system. While within a particular cell, a mobile telephone, or “handset”, can communicate with the base station providing the coverage for that cell. As the mobile telephone moves to the cell of another base station, communications between the mobile telephone and the base station providing the initial cell coverage can be transferred via a “hand off” to the other base station.
Each base station within a CDMA telephony system uses coded signals to communicate with mobile telephones. For example, typical CDMA telephony systems use pseudorandom number (PN) spreading codes, sometimes referred to as “short codes,” to encode data signals. These encoded data signals are transmitted to and from mobile telephones to convey digitized voice and/or other forms of communication. PN codes are known to those skilled in the art. The terms coded signals and encoded signals are interchangeably used herein.
To encode the data signals, the base station applies a PN code to the data at a rate faster than that of the data. For example, the PN code is applied to the data such that there are multiple “chips” of the code for any given element of data. Such an application of the PN code is commonly referred to as direct sequence spreading of the data. Chips and their associated chip rates are known to those skilled in the art.
Sometimes, each base station is assigned a particular timing offset of the short code to differentiate between base stations. Mobile telephones may therefore determine the identity of a particular base station based on the timing offset of the short code. Additionally, the data signals are often further encoded with a unique “covering” code. Such covering codes provide “channelization” for a signal that increases the number of unique communication channels. For example, data encoded with a covering code can further differentiate signals thereby improving detection and subsequent processing of a selected signal.
These covering codes are often used in CDMA telephony systems and typically include families of codes that are orthogonal (e.g., Walsh codes) or codes that are substantially orthogonal (e.g. quasi-orthogonal functions (“QOF”)). Orthogonal covering codes and QOF covering codes have properties that allow for the differentiation of unwanted signals and are known to those skilled in the art. Walsh codes are also known to those skilled in the art.
Both the short codes and the covering codes assist in the detection of a selected signal. However, interference caused by other signals may still degrade data extraction capabilities of the selected signal. For example, as a mobile telephone communicates with a particular base station within that base station's coverage cell, signals from other base stations can interfere with the mobile telephone communication. Since cells often overlap one another to ensure that all desired geographic regions are included in the communication system's coverage area, one or more signals from one base station may interfere with the communication link, or “channel,” between the mobile telephone and another base station. This effect is commonly referred to as cross-channel interference.
Cross-channel interference may also occur because some overhead channels are broadcast to all mobile telephones within the cell. These channels can “bleed” over into other cells and overpower a selected signal, thereby corrupting conveyed data. Examples of such channels include pilot channels, which are often broadcast at greater power levels and convey reference information and can be used to coherently demodulate other channels. Other potentially interfering channels may convey paging channels that alert a particular mobile telephone to an incoming call and synchronization channels that provides synchronization between a mobile telephone and a base station. Still other potentially interfering channels may include traffic channels bearing user traffic such as data and voice.
Still, other forms of interference may occur from “multipath” copies of a selected signal. Multipath can create interference because of the reception of copies of a selected signal at differing times. Multipath typically occurs because of obstructions, such as buildings, trees, et cetera, that create multiple transmission paths for a selected signal. These separate transmission paths may have unique distances that cause the signal to arrive at a receiver at differing times and is commonly referred to as co-channel interference. Additionally, these separate paths may bleed over into other cells to cause cross-channel interference.
Multipath creates co-channel interference because, among other reasons, the orthogonality of the covering code for a received signal is essentially lost due to timing offsets associated with the multipath. For example, a multipath signal having a covering code and arriving at a receiver at differing times causes a misalignment of the covering code. Such a misalignment can result in a high cross-correlation in the covering codes and a general inability to correctly retrieve conveyed data.
“Rake” receivers, such as those used in CDMA telephony systems, combine multipath signals to increase available signal strength. For example, a rake receiver may have a plurality of “fingers,” wherein each finger of the rake receiver independently estimates channel gain and other signal characteristics (e.g., phase) of the selected signal to more accurately demodulate data of the selected signal and subsequently retrieve the data. Each finger is assigned a particular “path” of the selected signal (i.e., one of the paths of the multipath signal or a signal from another base station). These paths may be combined to increase signal strength. Additionally, as signal characteristics change, the fingers may be assigned or de-assigned to other “paths” of the signal to improve data retrieval.
Rake receivers can improve data retrieval of a received signal. However, present rake receivers do not substantially reduce cross-channel interference and/or co-channel interference. These interferers may still corrupt data as long as they exist in any substantial form.